This invention relates to crankcase lubricating oil compositions. More specifically, the invention is directed to lubricating oil compositions that exhibit soot dispersing characteristics sufficient to allow the lubricating oil composition to pass an industry standard T8 test, with reduced levels of additive nitrogen.
Lubricating oil compositions comprise a major amount of a base oil and additives that improve the performance and increase the useful life of the lubricant. Nitrogen-containing dispersants are commonly used lubricant additives. The function of a dispersant is to maintain in suspension within the oil, insoluble materials formed by oxidation and other mechanisms during use of the oil, to prevent sludge flocculation and precipitation of the insoluble materials. Another function of the dispersant is to reduce the agglomeration of soot particles, thus reducing increases in the viscosity of the lubricating oil upon use. Crankcase lubricants providing improved performance, including improved soot dispersancy, have been continuously demanded.
To improve soot dispersancy, the industry has moved to the use higher molecular weight materials, which have superior dispersancy properties compared to lower molecular weight materials, and to use the high molecular weight dispersants in ever increasing amounts. However, dispersants are expensive. Further, common methods for forming high molecular weight, nitrogen-containing dispersants leave residual chlorine, which is introduced into the lubricant with the dispersant. The presence of chlorine leads to problems with the disposal of used lubricants, and lubricants containing reduced amounts of chlorine have been demanded. Also, greater levels of high molecular weight dispersant do not blend well in lubricants also containing overbased detergents. Still further, a high level of basic amine from dispersants contributes to the deterioration of seals within the engine during service. Therefore, it would be advantageous to provide a lubricant with adequate soot dispersancy properties using reduced amounts of high molecular weight, nitrogen-containing dispersant.
U.S. Pat. No. 1,815,022 to Davis (1931) discloses condensates of naphthalene and essentially linear chlorinated waxes formed by Freidel Craft alkylation of the naphthalene. Such compounds were described as functioning as wax crystal modifiers or lube oil flow improver (LOFI) additives and were added to oil to improve the cold flow characteristics thereof. These compounds have not been used for a number of years and, due to a high chlorine content, these compounds would be considered unsuitable for use in a modern passenger car, or heavy duty diesel motor oil formulations. In modern formulations, these compounds have been supplanted by fumarate/vinyl acetate copolymers or polymethacrylate-based LOFIs.
U.S. Pat. No. 4,708,809 to Davis describes a lubricating oil composition containing a phenolic compound of the formula:
(R)axe2x80x94Arxe2x80x94(OH)b 
wherein R is a saturated hydrocarbon group having 10 or more aliphatic carbon atoms; a and b are each independently 1 to 3 times the number of aromatic nuclei present in Ar; and Ar is a single, fused or linked polynuclear ring moiety that is optionally substituted. It is alleged that the addition of a minor amount of such a compound to a lubricant composition that is mixed with fuel will lead to a reduction in piston ring sticking in a two cycle engine.
The present invention, in brief summary, is directed to crankcase lubricating oils comprising a major amount by weight of an oil of lubricating viscosity; a minor amount by weight of a high molecular weight, nitrogen-containing dispersant; and an amount of an oil soluble, or oil dispersible aromatic hydrocarbyl oligomer sufficient to provide the lubricating composition with improved soot dispersing properties.